But the first new prescription weight-loss drug to reach the market in 13 years is having a hard time winning even a tiny slice of that huge market, despite an apparent need.

Sales of the drug, Qsymia (pronounced Kyoo-sim-EE-ah), have been minuscule since it went on sale last September. Sales totaled only $4.1 million in the first quarter of this year, even as Vivus, the manufacturer, spent $45 million on marketing, sales and administrative expenses.

Vivus’s stock price has plunged to $12.41 from $29 on the day after Qsymia was approved last July. And the company’s largest shareholder, saying the drug’s introduction was horribly botched, is battling to oust the entire board and top management at Vivus’s annual shareholder meeting on July 15.

While there are some problems unique to Qsymia, it is also facing barriers that are likely to confront other weight-loss drugs as well, including Belviq, a drug from Arena Pharmaceuticals and Eisai, that went on sale last month. Another drug, Contrave from Orexigen Therapeutics, is in advanced clinical trials.

These obstacles include lack of insurance reimbursement, modest weight loss, safety concerns, the troubled history of diet drugs, and a feeling on the part of many doctors and obese people themselves that excess weight is a lifestyle issue best addressed by more willpower, rather than a disease that requires medical treatment.

“You’ve got this turning of the battleship to change how the medical community views obesity,” said Dr. Barbara Troupin, vice president for scientific communications and risk management at Vivus.

The attitude is turning at least a little. Last month, the American Medical Association, the nation’s largest group of doctors, declared obesity a disease. In April, the American Association of Clinical Endocrinologists included weight-loss drugs in its diabetes treatment guidelines.

Given that one-third of American adults are obese and another third merely overweight, an effective and safe diet pill would seem like the path to instant riches. Jack Lief, the chief executive of Arena Pharmaceuticals, said in 2009 that sales of obesity drugs could eclipse those of statins, the cholesterol-lowering pills like Pfizer’s Lipitor, which had peak annual sales of about $13 billion before falling to generic competition.

Yet no prescription drug for obesity has ever reached annual sales of even $1 billion, the lower boundary of what the pharmaceutical industry considers a blockbuster.

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James Best Jr./The New York Times

While it is estimated that more than two million people take weight-loss drugs, the vast majority using the old generic appetite-suppressor phentermine, that is still only a tiny fraction of the 70 million or more obese adults.

And patients typically give up on the drugs quickly. An analysis of prescription data by the Food and Drug Administration found that only one quarter of people kept using a drug for at least three months, and only 10 percent for at least 180 days.

One reason is that many people don’t lose much weight. The clinical trials showed that those taking the recommended dose of Qysmia lost an average of 7.8 percent of their weight after a year. At that rate, a 250-pound person would end up at about 230 pounds.

Some people can do better than average, however. Marty McNamara, who started taking Qsymia in November, said he had dropped from 424 to 332 pounds as his appetite has virtually disappeared. Mr. McNamara, a 6-foot 5-inch highway maintenance worker from Ridgecrest, Calif., said he now eats only fruit for lunch.

“It’s amazing, because I like food,” he said, but quickly corrected himself. “I used to like food.”

Diet pills have been plagued by safety issues that have marred the image of the class. Two drugs used as part of the popular fen-phen combination were taken off the market in 1997 for damaging heart valves. In 2010, Abbott’s Meridia was withdrawn after a study suggested it might raise the risk of heart attacks and strokes.

Neither Qsymia nor Belviq has won approval in Europe, in part because of safety concerns. And Consumer Reports advises people to skip what it calls “quick-fix weight-loss drugs” because “their benefits are usually minimal and their adverse effects can be troublesome.” It urges people to lose weight the tried-and-true way, by dieting and exercising.

The main safety concern with Qsymia is that it might cause birth defects. For that reason, the F.D.A. required that the drug be distributed through only a handful of mail-order pharmacies, a cumbersome process for doctors and patients. Recently, however, the agency agreed that qualified retail pharmacies could also dispense the drug. Vivus said on Monday that Qsymia was now available through 8,000 retail drugstores, removing a major barrier to sales.

“I feel we have been held in check driving around the pit lane, without being able to go out and put this formula racecar to the test,” said Peter Y. Tam, the president of Vivus, which is based in Mountain View, Calif.

But there are still other roadblocks. Medicare Part D does not cover weight-loss drugs, although a bill was just introduced to change that. Only about one-third of Americans with private insurance have coverage for the drug, but often with co-payments of $50 a month or more. People without coverage will pay at least $150 a month.

“Most people don’t find that amount of money is worth it for that amount of weight loss,” said Dr. Ethan Lazarus, a weight-loss specialist in Denver. Dr. Lazarus said that only about five of his 600 active patients were on Qsymia.

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Terri Baker of Houston has not yet lost weight on Qsymia, but plans to continue with it. "Nothing else has worked," she said.Credit
Michael Stravato for The New York Times

Qsymia is a combination of two generic drugs, phentermine and topiramate. Dr. Lazarus said he sometimes prescribed the two generic drugs, which can cost as little as $40 a month.

The drug companies and other backers of the drug make the case that since obesity contributes to numerous other ailments like diabetes and heart disease, use of the pills to lower weight might actually produce a net savings for insurers, patients and Medicare.

Because of the cost and cumbersome distribution system, when Qsymia first went on sale, as many as 30 percent of the prescriptions were abandoned before being filled.

To counter that, Vivus is now offering the first two weeks of Qsymia free and is limiting out of pocket costs to $75 for the next month. It hopes that by the end of the first six weeks, people will have started to lose enough weight to stay on the drug.

Terri Baker of Houston is approaching that point now. Ms. Baker said she had not lost any weight yet, but said the drug made soda taste flat to her, allowing her to break her soft-drink habit. She plans to give the drug some more time, even though it will now cost her $150 a month.

“I really want to try this,” said Ms. Baker, who is 52 and weighs 200 pounds. “Nothing else has worked.”

Another problem for Qsymia is lack of resources. Vivus has only 150 sales representatives, far too few to reach primary-care physicians. Its disgruntled biggest shareholder, the First Manhattan Company, says one of Vivus’s biggest mistakes was not to enlist a big pharmaceutical company to help it sell the drug.

Vivus executives say they planned to target mainly endocrinologists first, but are now talking with larger pharmaceutical companies. They also say the company plans to start direct-to-consumer advertising in the fall.

Meantime, the company saysthat its strategy is falling into place, with retail pharmacies coming on line and organizations like the Veterans Health Administration and pharmacy benefits manager Express Scripts agreeing to pay for the drug.

Qsymia had been prescribed by 15,000 doctors to 40,000 patients as of the end of March, the company says, but prescriptions have risen since then.

Replacing the board with First Manhattan’s slate, it argues, would only “throw Vivus into turmoil at a critical juncture.”

First Manhattan, which owns 9.9 percent of Vivus, disagrees, saying in one letter to shareholders: “The light at the end of the current ‘stay the course’ tunnel is an oncoming train.”

A version of this article appears in print on July 2, 2013, on page B1 of the New York edition with the headline: Few Signs Of a Taste For Diet Pills. Order Reprints|Today's Paper|Subscribe